How can I improve viewport performance when using Dyntopo?
Here is an answer by Sanctuary from Dynatopo test thread:

Some performance tips about Dyntopo


Do not use Matcap, while it does not show while you're still blocking out your sculpt and you are in early stage of it, Matcap will very noticably impact performance when you're starting to reach mid/high poly . Use that addon instead , gives full control on the Blender existing opengl lights and does not impact performance.
Make sure you have Double Sided disabled in the Object Data panel, it's not a nvidia problem only, at higher poly stage the difference in performance with it being enabled or disabled is very noticable on every hardware.
Check the Detail level in the Dyntopo setting, too low (like less than 5) that can lead in a -very- lot of triangles generated by your brush strokes , while sometime you may want that, in other time that amount of tris could be really unneeded and can degrade performance. For most of the sculpt time getting detail between 15/20 is good, and for fine detailling, around 10 or a bit less should be enough (remember that when you zoom in you're fine detailling anyways) 
Less than 10 can be really overkill due the amount of generated tris.
You can optimise some part of your sculpt for which you think there are way too many triangles (enable Wire + Display All Edge to see what's going on while you sculpt) by either enabling the Collapse Short Edges button , increasing the detail setting and enabling for your brush the "Simplify" tool (in the bottom header Brush -> Sculpt Tool -> Simplify
You can press H and drag a selection to hide those parts while in Sculpt mode, it can help performance really a very lot depending on the kind of polycount you're having of course (ALT+H to unhide)
some people have mentionned that the regular freeze/pausing could be related to Blender autosaving (a function enabled by default for each 5 minutes) , you can disable or change the delay for that in File -> User Preferences -> File
additionally in freeze/pausing moment, clicking on "Optimise" in the Dyntopo settings may help so it recalculate whatever it has to recalculate
If rotating/zooming the view become painfully slow, press Z to switch to wireframe, rotating will then be very fast, press again Z to switch back to solid shading
Dyntopo can't currently reach (while keep performance good ) the same kind of polycount you can reach under Multiresolution, fortunately the "detail where needed" system of Dyntopo generating topology only where you work allows for detailled sculpt at a much lower polycount than an equivalent multires would require.
So don't be surprised if you can get a workable million in Multires and an unworkable million in Dyntopo.
Of course powerfull computers will get higher than less powerfull ones (and if i remember well, Dyntopo isn't multithreaded, while Multires is multithreaded).